Forget the Plaza duplex that Tommy Hilfiger put on the market for $50 million, and the $46.5 million Kress family’s penthouse, and Brooke Astor’s $46 million co-op, and the duplex that zinc magnate Bill Flaherty is listing for $43 million.

A source told The Observer yesterday evening that the monogram-shirted, meticulous uptown broker Edward Lee Cave is quietly taking offers on one of the most monumental apartments in New York. It’s Courtney Sale Ross’ duplex at 740 Park Avenue, the only residential building in New York with its own 559-page biography. Ms. Ross, a Texas-bred education philanthropist, is Time Warner creator Steve Ross’ widow.

Forget the Plaza duplex that Tommy Hilfiger put on the market for $50 million, and the $46.5 million Kress family’s penthouse, and Brooke Astor’s $46 million co-op, and the duplex that zinc magnate Bill Flaherty is listing for $43 million.

A source told The Observer yesterday evening that the monogram-shirted, meticulous uptown broker Edward Lee Cave is quietly taking offers on one of the most monumental apartments in New York. It’s Courtney Sale Ross’ duplex at 740 Park Avenue, the only residential building in New York with its own 559-page biography. Ms. Ross, a Texas-bred education philanthropist, is Time Warner creator Steve Ross’ widow.

Reached through his office, Mr. Cave was willing to briefly discuss the duplex, though he repeatedly said it isn’t openly on the market. “There will only be 10 people who can see the apartment. Because there will only be 10 people who are appropriate to see it,” he said. Just 10? “If somebody very, very, very important calls me,” he said, “I will run it by her.”

What’s happening here is called testing the waters–a phrase pronounced by the broker with his soft, careful Virginia accent–and it’s the proper first step to sell proper Upper East Side trophies. (Remember that last year, cell phone billionaire George Lindemann’s daughter Sloan Barnett and her husband Roger began quietly asking $62 million for their 125-year-old neo-Georgian townhouse on East 69th Street.)

At 740 Park, Ms. Ross and her late husband created the duplex out of two apartments. One reportedly had 14 rooms; the other, 18 (14 rooms plus 18 rooms equals 32 rooms). One of the apartment’s old owners told the writer Michael Gross that a dining room and kitchen were made into a children’s room, a living room became a screening room, and a bedroom was “sueded.”

Because the listing isn’t official, is there a tag? The broker would only say, “It’s going to be the most expensive apartment ever sold in New York.” The reported price for the most expensive residential deal (Harry Macklowe’s seventh-floor spread at The Plaza) is around $60 million. So will the duplex co-op sell for at least $60 million? “It’s not going to be ‘at least’ $60 million,” Mr. Cave offered. “It will be over $60 million.”

Uptown brokers like to say that there will never be a glut of inventory at the ridiculously high end of the market, simply because they’re dealing with people who would never have to downsize for financial reasons.